DEVELOPMENT & EVALUATION OF BIOLOGICAL CONTROL AGENTS FOR INVASIVE SPECIES THREATENING THE EVERGLADES & OTHER NATURAL AND MANANGED SYSTEMS
Location: Invasive Plant Research Laboratory
Title: A leaf-rolling weevil benefits from general saprophytic fungi in polysaccharide degradation
Submitted to: Arthropod-Plant Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 21, 2012
Publication Date: September 1, 2012
Citation: Li, X., Wheeler, G.S., Ding, J. 2012. A leaf-rolling weevil benefits from general saprophytic fungi in polysaccharide degradation. Arthropod-Plant Interactions. Volume 6, Issue 3(2012), page 417-424.
Interpretive Summary: Insects often feed on nutritionally poor diets. To compensate, they often employ the services of fungi that breakdown indigestible plant materials rendering these tissue more available for their digestion. A specialist weevil species Heterapoderopsis bicallosicollis is being considered as a biological control agent of the invasive weed Chinese tallow. The adult weevils of this species roll fresh leaf fragments into small bundle retreats in which they lay eggs before sealing. These bundles fall to the ground where they are exposed to many different species of fungus. These fungi digest the indigestible materials in the bundle leaves providing increased nutrition for the developing larvae. Experiments here show that these fungi decrease the amount of indigestible polysaccharides in these bundles. Moreover when fungal growth was inhibited, the insects had decreased weight gain, survival, and prolonged the larval stage. These results highlight the interdependence between this weevil and several species of fungi.
Insects form symbiosis with fungi widely, especially those feeding on leaf litter. As dead plant tissues provide poor quality diets which contain relatively high levels of indigestible lignin and cellulose, saprophytic fungi may increase nutrient availability by polysaccharide degradation. Although those inherited, obligate bacterial symbionts are well documented, those non-inherited, facultative fungal symbionts are relatively overlooked. The females of the leaf-rolling weevil Heterapoderopsis bicallosicollis, a specialist of Triadica sebifera, construct leaf-rolls that serve as retreats from which larvae feed internally. We found that fungi associated with leaf-rolls were not transported by the female but likely originated from the soil. To determine the effects of fungi on H. bicallosicollis development, fungal growth was reduced by a dry treatment. This treatment decreased adult weight and survival and prolonged larval duration significantly. We further tested the hypothesis that fungi degrade leaf-roll polysaccharides by a fungus inoculation experiment. Lignin, cellulose and soluble carbohydrate of leaf-rolls all decreased during fungi exposure. Three dominant fungi (Penicillium sp., Aspergillus sp. and Cladosporium sp.) decreased the levels of lignin, cellulose and soluble carbohydrate in inoculation experiments. We conclude that these saprophytic fungi form facultative associations with H. bicallosicollis and benefit weevil nutrition by polysaccharide decomposition. Our study highlights the significance of fungi in insect nutritional ecology.